(1) Field of the Invention
The present invention relates generally to measuring the dielectric constant of an unknown material and, more specifically, to a waveguide apparatus and method to thereby determine the dielectric constant.
(2) Description of the Prior Art
The dielectric constant of a material is an essential parameter that is important to know with confidence when doing electromagnetics work. This parameter, denoted εr, may be utilized to determine the wave number of an electromagnetic wave in a material and as a result, the phase velocity, guided wavelength, and so forth, of the electromagnetic wave.
Current methods for measuring the static dielectric constant include plating two opposite surfaces of a sample with conductive material and measuring the static capacitance that results. This is useful when the material is to be used for a capacitor or for low-frequency applications. This method may not be sufficiently informative when the material is to be used for RF applications because for most materials, the static (f=0 Hz) dielectric constant is different from that seen by a RF signal.
For RF purposes, measurement of the dielectric constant is often accomplished by use of an open-ended coaxial probe that is placed in contact with the sample and the impedance that is seen can be used to calculate εr. Measurement of the dielectric constant can also be performed by transmission of an RF signal through a plate of the unknown material in an anechoic environment, i.e., an environment that is free from echoes and reverberations. Another prior art method measures the frequency at which a block of the unknown material resonates at RF. In each of these prior art methods, a large sample of the material is required along with a suitable anechoic test environment.
The following patents discuss prior art attempts to solve problems related to the above:
U.S. Pat. No. 4,891,573, issued Jan. 2, 1990, to Gordon D. Kent, discloses a ceramic or other substrate, which is tested for dielectric constant K, and loss tangent by placing it on a central transverse plane across a cylindrical waveguide. A swept-frequency signal is injected into the waveguide at an input coupling loop and is picked up at an output coupling loop. Maximum transmission through the dielectric substrate occurs at a frequency that depends on the waveguide radius, the substrate thickness, and the dielectric constant. The dielectric constant can be obtained from the resonant frequency of a predetermined transmission mode. The loss tangent can be calculated from the transmission bandwidth. The measurement of the dielectric constant is insensitive to the position of the substrate in the gap between waveguide sections, and thus intimate contact is not required.
U.S. Pat. No. 4,996,489, issued Feb. 26, 1991, to P. L. Sinclair, discloses a system for measuring the complex dielectric constant of a core sample. The system incorporates a circular waveguide having a central axial transmitter coil. Equally spaced axial receiver coils are placed on both sides of the transmitter coil. The opposite polarity receiver signals are connected to an adder circuit to provide an output signal representing only the difference in the two received signals. By placing a standard, such as air, between the transmitter coil and one receiver coil, and a core sample positioned between the transmitter coil and the other receiver coil, the system obtains an output indicative of complex dielectric constant. Optionally, the system is operated in an oven to provide an elevated temperature, and can also be pressurized with a compressed fluid.
U.S. Pat. No. 5,001,433, issued Mar. 19, 1991, to S. Osaki, discloses apparatus and method for measuring electric characteristics of sheet-like materials using an instrument which includes a waveguide tube member having one end connected to transmitter for introducing a microwave into the tube member and the other end fully opened, a waveguide terminal member having an opened end facing the opened end of the tube member to form slit of the whole wave guide body constituted from the tube and terminal members and having the other end connected to first microwave detector, and an auxiliary waveguide branching from the wall portion of said tube member adjacent to the slit with the branch-extension end being associated with a second microwave detector.
U.S. Pat. No. 5,103,181, issued Apr. 7, 1992, to Gaisford et al., discloses radio frequency bridge techniques used to parameterize the complex dielectric properties of solids, liquids, gasses and mixtures thereof. This parameterization is performed in an electrically isolated, physically open structure, which allows continuous or batch monitoring of the materials and their mixtures. A method and apparatus are provided for measuring the composition of multi-component process streams flowing in pipes or ducts. The method uses the pipe in which the mixture flows as a waveguide in which propagating radio frequency electromagnetic energy is induced through dielectric loaded apertures. The dielectric measurement is performed in an electrically isolated, flow through test section that induces constructive or destructive interference patterns at characteristics frequencies. The characteristic frequency determines the dielectric constant of the mixture. The dielectric properties are used in turn to determine mixture composition. A density measurement is also provided for three component streams such as oil, water, and gas. Temperature and pressure measurements are made to correct for temperature and pressure induced variations in calibrated component impedance and density values.
The above cited prior art does not provide an apparatus and method utilizing an air-filled metallic waveguide fitted with a metal (e.g. brass) septum or plate that divides the waveguide in half for a portion of its length whereby the material to be measured is fitted in the waveguide on either side of the metal sheet. The above-cited prior art does not show a tapered or restricted diameter waveguide that may be utilized with a single sample of the material without requiring a metal sheet. The above-cited prior art does not utilize a waveguide section operated in a cutoff or evanescent mode. Moreover, it would be desirable to be able to measure the dielectric constant of a sample material without the need to accurately measure the length of the sample.
The solutions to the above-described problems are highly desirable but have never been obtained or available in the prior art. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.